一个液压系统压力低时：（）.
A.主警告灯亮
B.主警告灯亮且“叮”的一声
C.无警告
参考答案：B
E-mail, as well as telephones, ______ an important part in daily communication.
A．is playing
B．have played
C．are playing
D．play
“以知识为本”使教育异化为记忆和训练内容。
